Exposure meter

ABSTRACT

An exposure meter which includes a Wheatstone Bridge circuit having first and second house sides. One house side includes a photoconductive cell and a plurality of resistors which can be selectively and individually connected in series with the photoconductive cell at any of a series of illumination values of light incident thereon so that the normally nonlinear relationship between the resistance of a photo conductive cell and the illumination of light incident thereon can be converted to a useable relationship for that specific series. The second house side includes a resistance size (increasing f/stop number) as the ladder is climbed, and two pairs of resistance ladders, one pair representing the increasing camera speed (FPS or frames per second) as the ladders are climbed and one pair representing the increasing film sensitivity (ASA rating) as the ladders are climbed. The individual ladders of each pair are identical, have a preselected number of steps and are conjointly set to the same setting or position so that when one ladder of each pair is interconnected and each so connected pair is respectively connected to the top or bottom of the first resistance ladder a constant total resistance will be maintained, for all possible settings, on the second house side. The Bridge can be standardized so that the photoconductive cell can be calibrated and compensated for any deviation from a standard set of resistance values for the specific illumination series. Also provided is means for multiplying each of the camera speed (FPS) setting by a predetermined multiple so that a wide range of FPS settings can be conveniently provided.

United States Patent J antzen et al.

[ 1 Sept. 26, 1972 154] EXPOSURE METER [72] Inventors: Charles A.Jantzen, 190 Alps Road,

Wayne, NJ. 07470; Victor J. Canziani, 1145 E. 21 St., Brooklyn, N.Y.11210 [22] Filed: Aug. 30, 1971 [21] Appl. No.: 175,838

[52] US. Cl. ..356/226, 95/10 C, 356/224, 356/228 [51] Int. Cl ..G0lj1/44, 001 1/42 [58] Field of Search ..356/218, 223, 224, 225, 226,356/227, 228; 95/10 C [56] References Cited UNITED STATES PATENTS2,703,035 3/1955 Tobler ..356/226 Primary Examiner-Ronald L. WibertAssistant Examiner-V. P. McGraw Attorney-Robert E. Isner et al.

[ 5 7 ABSTRACT An exposure meter which includes a Wheatstone Bridgecircuit having first and second house sides. One house side includes aphotoconductive cell and a plurality of resistors which can beselectively and individually connected in series with thephotoconductive cell at any of a series of illumination values of lightincident thereon so that the normally nonlinear relationship between theresistance of a photo conductive cell and the illumination of lightincident thereon can be converted to a useable relationship for thatspecific series. The second house side includes a resistance size(increasing f/stop number) as the ladder is climbed, and two pairs ofresistance ladders, one pair representing the increasing camera speed(FPS or frames per second) as the ladders are climbed and one pairrepresenting the increasing film sensitivity (ASA rating) as the laddersare climbed. The individual ladders of each pair are identical, have apreselected number of steps and are conjointly set to the same settingor position so that when one ladder of each pair is interconnected andeach so connected pair is respectively connected to the top or bottom ofthe first resistance ladder a constant total resistance will bemaintained, for all possible settings, on the second house side. TheBridge can be standardized so that the photoconductive cell can becalibrated and compensated for any deviation from a standard set ofresistance values for the specific illumination series. Also provided ismeans for multiplying each of the camera speed (FPS) setting by apredetermined multiple so that a wide range of FPS settings can beconveniently provided.

8 Claims, 5 Drawing Figures PATENTEUSEPZB I972 SHEET 2 BF 3 INVENTORSCHARLES A. JANTZEN VICTOR J. CAN ZIANI ATTORNEY EXPOSURE METER Thepresent invention relates to exposure meters and more particularly toexposure meters specifically intended for use with high speed rotatingprism cameras, but is adaptable to almost any photographic camera.

When motion photographs are taken it is essential to good photographythat a satisfactory adjustment be attained amongst the four principalcontrol parameters, i.e., light, film speed, camera exposure time anddiaphragm opening.

Traditionally, personal experience is the keystone for determining theabove referred to satisfactory adjustment of the four principal controlparameters and as a result, on occasion, even the most experiencedphotographer will misjudge a given situation and the exposure of thedeveloped film will not be satisfactory.

With the complexity and sophistication of experiments, investigations,studies or recording relying on movie camera apparatus to make apermanent analyzable record thereof, increasing with the concomitantincrease in cost for such experiments and the like it becomes mostdesirable for these experiments to be properly recorded the first time.

It is accordingly the principal objective of the present invention toprovide an exposure meter for determining when a good combination of theabove control parameters has been achieved whereby the possible errordue to humanjudgment can be minimized.

It is a further objective of the present invention to provide anexposure meter wherein a given photoconductive cell can be calibrated inorder that a high degree of accuracy can be achieved.

It is another objective of the present invention to provide such anexposure meter wherein the calibration of the exposure meter can bequickly and accurately checked.

Among the advantages of the present invention is the provision of anexposure meter wherein several suitable combinations of the basic fourparameters can be defined whereby the most satisfactory solution can beutilized.

Other objectives and advantages of the present invention will becomeapparent from the following portion of this specification and from theaccompanying drawings which illustrate in accord with the mandate of thepatent statutes a presently preferred embodiment incorporating theprinciples of the invention.

Referring to the drawings:

FIGS. and lb are a circuit diagram of the exposure-meter of the presentinvention.

FIG. 2 is a graphical representation of the relationship between theresistance of a conventional photoconductive cell and the illuminationof light incident thereon.

FIG. 3 is a graphical representation similar to that of FIG. 2additionally illustrating the same relationship between photoconductivecells which have a deviation from the standard photoconductive cell overthe normal operating range.

FIG. 4 is a graphical representation of a deviation curve for aphotoconductive cell.

The circuit diagram for the presently preferred embodiment of theexposure meter is illustrated in FIG. 1 and the invention will bedescribed with specific reference to the individual circuits thereof.

2 LINEARIZATION CIRCUIT The basic circuit for the exposure meter is aWheatstone Bridge which includes first and second arms connected attheir common ends to a source of constant voltage A. The first arm,which includes several known, fixed resistances of appropriate valuesplus the photoconductive cell, defines one side of the house. The secondarm, which includes first and second resistances or more resistanceswhich can be varied through a predetermined range and through which aconstant current flows, defines the second house side.

One of the known and fixed resistances of the first arm is a resistor R,across which resistance are developed signal voltages in proportion tothe illumination incident on the photoconductive cell and which signalsare inserted from the left house side into the Bridge for analysis.Another one of the resistances of the first arm is a photoconductivecell PC which preferably is to be positioned at the focal plane of acamera to sense the amount of light incident thereon.

The resistance of a photoconductive cell varies nonlinearly withillumination. An illustrative curve of resistance R,, versusillumination for a particular type of photoconductive cell isillustrated in FIG. 2.

In order that a proportional relationship exists between the intensityof the light incident on the photoconductive cell and the currentallowed to flow through the first arm of the bridge for a predeterminedseries of conventional illumination levels, for example: a footcandleseries (F5 1), (b=%), (c=l), (d=2), (3 =4 ),(f=8), (g=l6), (h=32),(i=64), (j=l25), (K=250), a resistor R n, R having a selected value isadded to the resistance of the photoconductive cell at each of the aboveratings in addition to a fixed resistance of R, continuously employedwith the photoconductive cell and R In the preferred embodimentillustrated in FIG. 1, each of these resistors R may be selectively andindividually connected in series with the photoconductive cell by meansof suitable switches SW4B and SW7.

OPERATING CIRCUIT The second arm of the basic Wheatstone Bridge circuitincludes a ladder of resistors D having a number of steps (which may beequal) or resistors R R equal to the number of and corresponding torespective resistors R R of the linearizing circuit and representing thediminishment of the diaphragm opening, a pair of identical resistorladders F PS, F PS each having a plurality of identical resistors R R Rrepresenting the increase in the camera speed (FPS) and a pair ofidentical resistor ladders ASA ASA having a series of resistors R R1500representing the increasing sensitivity of film as the ASA ratingprogresses along a conventional rating sequence.

The setting of diaphragm/light switch SW4A to a specific setting, as forexample at the 175.6 setting or number 6 position, determines thelocation on the second house side where the null meter joins the secondarm. The setting of frames per second (FPS) switch SWSB, which is setconjointly with similar switch SW5A, defines the desired setting for theframes per second or camera speed, and the setting or film speed switchSW6B, which is set conjointly with similar switch SW6A, defines thedesired setting for the film speed (ASA).

As can be appreciated by the utilization of pairs of identical laddersfor representing camera speed and film speed, by switching each ladderof a given pair in an identical manner, and by defining upper and lowerresistances which complementally represent the total resistance of anindividual ladder of the pair, the total resistance of the second armfrom the positive side of the battery A to the negative side of thebattery A will always be constant, and accordingly the current flowingthrough the second arm will be constant for all possible settings.

Prior to operating the exposure meter the subject, camera, accessories,lights and lens (all not shown) are readied in the usual manner. Thecamera lens is focused with the diaphragm wide open at the largestf/stop setting and the photoconductive cell is preferably positioned atthe focal plane of the camera so as to read the light as the film in thecamera sees it.

The exposure meter is then operated by setting the conjointly operablefunction switches SW2A, SW2B, SW2C, SW2D to operate or position number2, setting the conjointly operable mode switches SW3A, SW38, SW3C, SW3Dto exposure or position number 2, setting the conjointly operable filmspeed switches SW6A, SW6B to the ASA rating of the film to be used (ornearest position), setting the conjointly operable frames per secondswitches SWSA, SWSB to the desired frames per second camera speed (ornearest position) while setting the frames per second multiplyingswitches SW8A, SW8B conjointly to position 1 and settingthediaphragm/light switch SW4A to the f/stop position nearest to thenatural aperture of the camera. Balance switch SW7 is then set at thestep corresponding to the step at which switch SW4A is set. Actuateswitches SWlA and SWlB are now conjointly actuated, and if the readingof the galvanometer or null-indicating meter is as close to zero as ispracticable, a good combination of the four variables (light, filmspeed, frames per second and f/stop) is indicated.

Should the null meter read positive or to the right of zero, excessivelight is indicated. Accordingly, several adjustments either individuallyor in combination can be made, such as: the setting of diaphragm of thecamera objective lens can be adjusted to a smaller f/stop, less lightcan be put on the subject, film with a lower ASA rating can be utilizedor a higher camera speed can be selected. Adjustments are thus effecteduntil the null meter reads as close to zero as is practicable.

On the other hand, should the null meter read negative (to the left ofzero) insufficient light is indicated and accordingly either more lightcan be put on the subject or a film with a higher ASA rating can beutilized or a lower camera speed can be selected. Adjustments are thuseffected until the null meter reads as close to zero as is practicable.

The photoconductive cell is then removed from the camera and the cameracan be operated with film of the indicated ASA rating, at the indicateddiaphragm opening and at the indicated frames per second.

As previously noted the nonlinear output of the photoconductive cell waslinearized. This was done since it would be an extremely complex matterto balance the Wheatstone Bridge since the variation of the diaphragm,FPS and ASA ladders would have to be matched not only to the nonlinearvariations of the photoconductive cell but to each other as well.

STANDARDIZING CIRCUIT In order to attain extremely accurate results theactual resistance R R curve of any photoconductive cell pc, pc" shouldtrace, as closely as possible, the nominal curve R illustrated in FIG.2. To this end the right side of the house is standardized to a knownstandard, the left side of the house is step-standardized and includinga photoconductive cell is calibrated to determine whether and to whatextent the actual curve for the photoconductive cell varies from thenominal curve, and the left side of the house is suitably compensated soas to conform, as best as possible, the actual to the nominal curve.

Jumper connectors A, B, C, D provide circuits availability andcapability in the left side of the house for calibration compensations.Initially, and until later calibrations indicate otherwise, jumperconnectors A, B are joined to produce a closed circuit for a nominalcurve photoconductive cell and to complete internal circuitry of theexposure meter for operation.

To standardize the second house side to a known standard, a WheatstoneBridge Circuit is established by switching function switches SW2A, SW28,SW2C, SW2D to their number 3 positions, by switching mode switches SW3A,SW38, SW3C, SW3D to their number one or light positions, whereby astandard resistor R which is equivalent to the total of the constantresistance of the complemental FPS and ASA ladder flow paths and theresistance of a multiplying circuit resistor R is switched into thecircuit of the right house side in place of the FPS and ASA ladders andthe multiplying circuit.

The photoconductive cell is now replaced by a resistor whose resistanceis equal to the stated resistance of the photoconductive cell at a givenillumination (8 foot-candles) and the corresponding linearizingresistance R is placed in series therewith by thy selective positioningof diaphragm/light switch to the 8 footcandles setting, whichpositioning also admits the D ladder selective point of switch SW4A forthe null meter.

Thus the configuration of both sides of the house in both arms of theBridge are selectively arranged to permit standardization to beperformed.

The primary battery A is actuated by closing actuate switch SWlA and theright side of the house is standardized to the left side by adjusting avariable resistor R which may be in the form of a potentiometer orrheostat, until the null meter is nulled.

Such standardization is performed with the exposure meter attuned in thelight mode as a light-measuring meter.

STEP-STANDARDIZING CIRCUIT After the second side of the house has beenstandardized as above, only then can the first side of the house bestep-standardized by adjusting its step circuit.

To step-standardize the first side of the house, the standard resistor Ris switched out and the complemental FPS and ASA flow paths and theresistance of a switches SWSA, SWSB and the film speed switches SW6A,SW6B are switched to standard settings, for example: f5.6, 8, 800 fps,200 ASA respectively.

The photoconductive cell is now replaced by the same resistor, whichserved in standardizing the right house side, and the same correspondinglinearizing resistance R is placed in series therewith by the selectivepositioning of the balance switch to the 8 foot-candle setting.Positioning of switch SW4A admits the D ladder selective point for thenull meter.

Concurrently by the positioning of switches SW2D and SW3B,anpotentiometer R is entered into the circuits of the left house side.

Thus the configurations of both sides of the house in both arms of theBridge are selectively arranged to permit step-standardization to beperformed.

The secondary battery B is actuated by closing actuate switch SWlB,which closes conjointly with switch SWlA, and the left side of the houseis step-standardized to the right side, which had to be previouslystandardized, by adjusting potentiometer R until the null meter isnulled.

Such step-standardization is performed with the exposure meter attunedin the exposure mode, and its success is dependent upon prior successfulstandardization being performed.

CALIBRATING CIRCUIT Standard photoconductive cells may have asubstantial deviation from their stated nominal values. The solid lineidentified R (FIG. 3) represents the nominal or standard curverepresenting the relationship between the illumination sensed by thephotoconductive cell and the resistance of the photoconductive cellwhereas the dotted lines R R represent illustrative maximum and minimumdeviation curves for such a photoconductive cell.

To determine whether and to what extent a given photoconductive celldeviates from the standard curve illustrated in FIG. 3, functionswitches SW2A, SW2B, SW2C, SW2D are switched to position number 2, modeswitches SW3A, SW3B, SW3C, SW3D are switched to position two, FPSswitches SWSA, SWSB are switched to standard setting number 4 (800 FPS),ASA switches SW6A, SW68 are switched to standard setting number 4(200ASA), multiplier switches SW8A, SW8B are switched to position one andbalance switch SW7 and diaphragm switch SW4A are systematically andconcurrently switched from position one to position 11 as sources ofknown illumination corresponding to the setting of switch SW7 arepositioned adjacent to the photoconductive cell whereby the positive ornegative deviation from null for the photoconductive cell will beindicated on the null meter. (Alternate means of evaluating positive ornegative deviation from null as indicated on the null meter for thephotoconductive cell may be used by switching mode switches SW3A, SW3B,SW3C, SW3D to position one and conjointly switching switches SW4A, SW4Bsystematically from position one to position 1 l as sources of knownillumination corresponding to the setting of switch SW4B are positionedadjacent to the photoconductive cell.)

An illustrated graph of the deviation of a specific photoconductive cellat constant ASA and FPS ratings (200 ASA, 800 FPS) is illustrated inFIG. 4. To minimize the error inherent in the utilization of the givenphotoconductive cell, the curve is effectively shifted by effectivelylowering this graph Rmdev) by a selected amount, by designed means ofcircuit applications and controls available by design within theexposure meter at given reference illuminations so that withinreasonable limits the effective graph will approach or straddle thenominal curve with a minimum positive and negative deviation.

This compensation can be effected by utilization of a compensatingcircuit 20. This circuit includes a secondary voltage source B, variableresistors R R which may be potentiometers, standard resistor R andjumper connectors A, B, C, D. To effectively reduce the resistance ofthe photoconductive cell and hence effectively the resistance of thefirst arm, jumper connectors A and D are joined, jumper connectors A, Bare disjoined and potentiometer R is adjusted with the diaphragm/lightand balance switches set to the illumination setting by placing apredetermined additive or positive voltage step in series with the firsthouse side. When it is ascertained that calibration of a photoconductivecell requires the joining of jumper connectors A and D,step-standardization must be performed again to compensate itsadjustment for the joined jumper connectors A, D circuit configuration,and the final calibration of the photoconductive cell then is made byadjusting variable resistor R To effectively increase the resistance ofthe photoconductive cell, jumper connectors A and B are joined, jumperconnectors C and D are joined, jumper connectors A, D are disjoined ifjoined and potentiometer R is adjusted with the diaphragm/light andbalance switches set to the illumination setting by placing apredetermined subtractive or negative voltage step in series with thefirst house side. If the actual photoconductive cell curve effectivelyfollows the standard or nominal curve, no compensation is required andonly jumper connectors A and B are joined.

To ascertain whether the once calibrated meter has gone out ofadjustment, the standardization circuit may be re-established at anytime to determine whether the once established standardization hasremained unchanged (null meter reading at or near zero) and then thestep-standardization circuit can be reestablished with the FPS laddersFPS FRS the ASA ladders ASA,, ASA and the diaphragm opening ladder D andoptional FPS multiplier set to the above referred to standard settingsto determine whether the once established equality between the first andsecond house sides has remained unchanged (null meter reading at or nearzero). If the system has gone out of adjustment the above procedures forstandardizing, step-standardizing, calibrating and compensating theexposure meter will be readily repeated.

To increase each FPS setting by ll times a multiplier 30 is provided.Switching the multiplier switches SW8A, SW8B to position number two willincrease each FPS setting 1% times. Such a multiplier enables theexposure meter to have a broad FPS range with small increments. Themultiplier needs not be limited to 1% times but may be established atother multiplier levels by judicious use of appropriate values for themultiplier resistors.

To use the apparatus as a light meter for light levels up to 250-footcandles at the camera focal-plane, mode switches SW3A, SW3B, SW3C, SW3Dare set to light or number one position and diaphragm/light switch SW4Ais adjusted until the null meter is nulled. The foot-candle reading ofthe diaphragm/light switch will then indicate the light level.

BATTERY CHECK BATTERY A A first battery A provides a source of constantvoltage for a Wheatstone Bridge circuit and a cheking circuit for thisbattery is defined by switching actuate switch SWlA to the closedposition and by switching function switches SW2A, SW2B to their numberfour positions. The checking circuit includes the null meter G, aresistor R and the voltage source A. The resistance of the resistor isselectively chosen so that if the voltage source is developing a voltagewithin its normal operating range the null meter will register a currentflow within a selected range indicated by the null meter.

BATTERY CHECK BATTERY B A second voltage source in the form of a batteryB is connected to a compensating circuit which will be subsequentlydiscussed in detail. The circuit for checking the output of the secondvoltage source B is defined by switching actuate switch SWlB, which isswitched conjointly with actuate switch SWlA, to closed position and byswitching function switches SWZA, SW2B to their number five positions.This checking circuit includes the null meter G and a resistor R ofselected value and the voltage source B. When the voltage source B isdeveloping a voltage within a predetermined range the null meter willregister a current reading within a selected range thereby providing acheck on this battery.

PROTECTIVE CIRCUIT To protect the galvanometer or null meter duringtransit and non-use a protective circuit can be established by switchingfunction switches SWZA, SWZB to position number 1 whereby thegalvanometer will be completely isolated and shunt dampened.

Having thus described our invention, we claim:

1. An exposure meter comprising Wheatstone Bridge circuit meansincluding first and second house sides, a voltage source and agalvonometer,

said first house side including a photoconductive cell having aresistance which varies non-linearly with the illumination of lightincident thereon,

means for linearizing the relationship between the resistance of saidphotoconductive cell and the illumination of light incident thereon fora preselected series of footcandle valves,

resistance means for developing signal voltages for the Bridge inproportion to the illumination incident on said photoconductive cell,said second house side including a first resistance ladder having aplurality of resistive steps representing the size of the diaphragmopening in a camera for a selected series of diaphragm openings,

connecting means for selectively connecting one side of saidgalvanometer to said first ladder at any selected step thereof wherebyfirst upper and lower resistances are defined,

a first pair of identical resistance ladders each having a plurality ofresistive steps and representing a selected series of frames per secondcamera speeds,

a first pair of conjointly operated connecting means,

one of said pair connecting the top of said first resistance ladder to aselective step of one of said first pair of resistance ladders and theother one of said pair connecting the bottom of said first resistanceladder to the same selective step of the other one of said first pair ofresistance ladders,

a second pair of identical resistance ladders each having a plurality ofresistive steps and representing a selected series of film speeds,

a second pair of conjointly operated connecting means, one of said pairconnecting the top of said one of said first pair of resistance laddersto a selected setting of one of said second pair of resistance laddersand the other one of said second pair of connecting means connecting thebottom of said other one of said first pair of resistance ladders to thesame selected setting of the other of said second pair of resistanceladders, and

means for connecting one end of one of said second pair of resistanceladders to one side of said voltage source and for connecting the otherend of the other of said second resistance pair to the other side ofsaid voltage source.

2. An exposure meter according to claim 1 wherein said second house sidefurther comprises means having a predetermined resistance forselectively multiplicitively increasing the value of each of said cameraspeed settings by a predetermined amount.

3. An exposure meter according to claim 2 further comprising a firststandard resistance of preselected value representing a standard valuefor said photo-conductive cell at a given illumination, means forremoving said photoconductive cell from said one housing side andsubstituting said first standard resistance therefor.

4. An exposure meter according to claim 3, further comprising a secondstandard resistance of preselected value representing the cumulativeresistance of one of said first and second pairs of resistance laddersand said multiplying means and means for removing said first and secondpairs of resistance ladders and said multiplying means from said secondhouse side and substituting said second standard resistance therefor. I

5. An exposure meter according to claim 4 wherein said second house sidefurther comprises variable resistor means for standardizing said secondhouse side including said second standard resistance to said first houseside including said first standard resistance.

6. An exposure meter according to claim 5, further comprising firstmeans for stepping up the voltage of ductive cell.

8. An exposure meter according to claim 7, further comprising secondmeans for selectively stepping up the voltage of said first house sideto compensate for a photoconductive cell having a higher series ofresistance readings over the selected series of candlepower ratings thanthe resistance readings for a standard photoelectric cell.

1. An exposure meter comprising Wheatstone Bridge circuit meansincluding first and second house sides, a voltage source and agalvonometer, said first house side including a photoconductive cellhaving a resistance which varies nonlinearly with the illumination oflight incident thereon, means for linearizing the relationship betweenthe resistance of said photoconductive cell and the illumination oflight incident thereon for a preselected series of footcandle valves,resistance means for developing signal voltages for the Bridge inproportion to the illumination incident on said photoconductive cell,said second house side including a first resistance ladder having aplurality of resistive steps representing the size of the diaphragmopening in a camera for a selected series of diaphragm openings,connecting means for selectively connecting one side of saidgalvanometer to said first ladder at any selected step thereof wherebyfirst upper and lower resistances are defined, a first pair of identicalresistance ladders each having a plurality of resistive steps andrepresenting a selected series of frames per second camera speeds, afirst pair of conjointly operated connecting means, one of said pairconnecting the top of said first resistance ladder to a selective stepof one of said first pair of resistance ladders and the other one ofsaid pair connecting the bottom of said first resistance ladder to thesame selective step of the other one of said first pair of resistanceladders, a second pair of identical resistance ladders each having aplurality of resistive steps and representing a selected series of filmspeeds, a second pair of conjointly operated connecting means, one ofsaid pair connecting the top of said one of said first pair ofresistance ladders to a selected setting of one of said second pair ofresistance ladders and the other one of said second pair of connectingmeans connecting the bottom of said other one of said first pair ofresistance ladders to the same selected setting of the other of saidsecond pair of resistance ladders, and means for connecting one end ofone of said second pair of resistance ladders to one side of saidvoltage source and for connecting the other end of the other of saidsecond resistance pair to the other side of said voltage source.
 2. Anexposure meter according to claim 1 wherein said second house sidefurther comprises means having a predetermined resistance forselectively multiplicitively increasing the value of each of said cameraspeed settings by a predetermined amount.
 3. An exposure meter accordingto claim 2 further comprising a first standard resistance of preselectedvalue representing a standard value for said photo-conductive cell at agiven illumination, means for removing said photoconductive cell fromsaid one housing side and substituting said first standard resistancetherefor.
 4. An exposure meter according to claim 3, further comprisinga second standard reSistance of preselected value representing thecumulative resistance of one of said first and second pairs ofresistance ladders and said multiplying means and means for removingsaid first and second pairs of resistance ladders and said multiplyingmeans from said second house side and substituting said second standardresistance therefor.
 5. An exposure meter according to claim 4 whereinsaid second house side further comprises variable resistor means forstandardizing said second house side including said second standardresistance to said first house side including said first standardresistance.
 6. An exposure meter according to claim 5, furthercomprising first means for stepping up the voltage of said first houseside for step standardizing said first house side including said firststandard resistance to said second house side including said first andsecond resistance pairs and said multiplying means.
 7. An exposure meteraccording to claim 6 further comprising means for selectively steppingdown the voltage of said first house side to compensate for aphotoconductive cell having a lower series of resistance readings overthe selected series of illumination ratings than the resistance readingsfor a nominal photoconductive cell.
 8. An exposure meter according toclaim 7, further comprising second means for selectively stepping up thevoltage of said first house side to compensate for a photoconductivecell having a higher series of resistance readings over the selectedseries of candle-power ratings than the resistance readings for astandard photoelectric cell.